Dimethylbiphenyl (DMBP) and other dialkylbiphenyls are useful intermediates in the production of a variety of commercially valuable products, including polyesters and plasticizers for PVC and other polymer compositions. For example, DMBP can readily be converted to an ester plasticizer by a process comprising oxidation of the DMBP to produce the corresponding mono- or dicarboxylic acid followed by esterification with a long chain alcohol. However, for certain uses, it is important to reduce the level of 2,X′ DMBP (where X′ is 2′, 3′ and 4′) isomers in the product since, for example, diphenate esters having substitution on the 2-carbons tend to be too volatile for use as plasticizers.
In addition, 4,4′-diphenyl-dicarboxylic acid, optionally together with diphenyl-3,4′-dicarboxylic acid, is a potential precursor, either alone or as a modifier for polyethylene terephthalate (PET), in the production of polyester fibers, engineering plastics, liquid crystal polymers for electronic and mechanical devices, and films with high heat resistance and strength.
Homopolyesters of 4,4′-biphenyl dicarboxylic acid (BDA) and various aliphatic diols have been disclosed in the literature. For example, in the Journal of Polymer Science, 9, 35 (1952), Ezard discloses homopolyesters of 4,4′-biphenyl dicarboxylic acid and ethylene glycol. Similarly, in the British Polymer Journal, 13, 57 (1981), Meurisse et al. disclose homopolyesters made from 4,4′-biphenyl dicarboxylic acid and a number of diols including ethylene glycol, 1,4-butanediol and 1,6-hexanediol. Homopolyesters of 4,4′-biphenyl dicarboxylic acid and ethylene glycol are also disclosed in, for example, U.S. Pat. Nos. 3,842,040 and 3,842,041.
Copolyesters of 4,4′-biphenyl dicarboxylic acid with mixtures of aliphatic diols are also disclosed in the literature, see for example, in U.S. Pat. No. 2,976,266. In addition, in U.S. Pat. No. 4,959,450, Morris et al. disclose copolyesters from 4,4′-biphenyl dicarboxylic acid and mixtures of 1,4-cyclohexanedimethanol and 1,6-hexanediol. Copolyesters of 4,4′-biphenyl dicarboxylic acid and terephthalic acid with certain aliphatic diols are also disclosed in the literature, for example, in the Journal of Polymer Science, Polym. Letters, 20, 109 (1982) by Krigbaum et al. Moreover, U.S. Pat. No. 5,138,022 discloses copolyesters of 3,4′ biphenyl dicarboxylic acid and optionally 4,4′-biphenyl dicarboxylic acid, and certain aliphatic diols, like ethylene glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol.
As disclosed in our co-pending U.S. patent application Ser. Nos. 14/201,287 and 14/201,224, both filed Mar. 7, 2014, dimethyl biphenyl may be produced by hydroalkylation of toluene followed by dehydrogenation of the resulting (methylcyclohexyl)toluene (MCHT). However, even using a selective molecular sieve catalyst for the hydroalkylation step, this process tends to yield a mixture of all six DMBP isomers, namely 2,2′, 2,3′ 2,4′, 3,3′, 3,4′ and 4,4′ DMBP, in which the 2,X′ (where X′ is 2′, 3′ or 4′) and 3,3′ DMBP isomer content may be 50% by weight or more of the total DMBP product. The entire disclosures of U.S. patent application Ser. Nos. 14/201,287 and 14/201,224 are incorporated herein by reference in their entirety.
Alternative routes to DMBP via benzene are described in co-pending U.S. patent application Ser. No. 14/164,889, filed Jan. 27, 2014, in which the benzene is initially converted to biphenyl, either by oxidative coupling or by hydroalkylation to cyclohexyl benzene (CHB) followed by dehydrogenation of the CHB, and then alkylation of the biphenyl with methanol. Again, however, the alkylated product is a mixture of DMBP isomers, in which the levels of the desired 3,4′ and 4,4′ isomers may be lower than 50% by weight of the total DMBP product.
Other references of interest include: U.S. Pat. No. 8,829,093; U.S. patent application Ser. Nos. 14/486,945, 14/480,363, 14/527,480, 13/316,745, 62/012,024, 62/012,037, and 62/068,144; and U.S. Patent Publication Nos. 2014-0212666, 2014-0316155, 2014-0275606, 2014-0275609, and 2014-0275605.
There is, therefore, interest in developing a process for producing dialkyl-substituted biphenyl compounds in which the yield of the 3,4′ isomer, and particularly the 4,4′ isomer, is maximized. At the same time, for acceptable process carbon efficiencies, it is important to utilize the 2,X′ isomer content. In other embodiments, it may be desirable to enhance the concentration of other isomers, even the 2,X′ isomers, in a mixture of dialkyl-substituted biphenyl compounds.